Server Side Includes allow for your server to parse your html pages and return certain things in place of your server side include calls.

Long Answer:

Server Side Includes allow you to imbed special tags in order for your server to parse this document and replace these tags with the information you requested to beplaced there, whether it be a CGI script, command, or even an environment variable. This all occurs on the server side, which means that when the document reaches the end user, they won't even know the server side includes were ever used, and if they view source, they won't see any trace of them.